Washington, Aug 27 (IANS): US President Donald Trump has called for drug tests to be administered before his first presidential debate with Democratic candidate Joe Biden next month, it was reported.

Trump made the remarks in an Oval Office interview with the Washington Examiner magazine on Wednesday, saying he noticed a sudden improvement in Biden's primary debate performance against Senator Bernie Sanders in March, reports The Hill news website.

The President however, offered no evidence to support his suggestion that the improvement could have been the result of drugs.

Biden participated in 11 primary debates, most of which were against a crowded field of other contenders, but the last one was on March 15 against just Sanders.

"Nobody thought that he was even going to win," Trump said.

"Because his debate performances were so bad. Frankly, his best performance was against Bernie. We're going to call for a drug test, by the way, because his best performance was against Bernie.

"I don't know how he could have been so incompetent in his debate performances and then all of a sudden be OK against Bernie.

"My point is, if you go back and watch some of those numerous debates, he was so bad. He wasn't even coherent. And against Bernie, he was. And we're calling for a drug test," The Hill news quoted Trump as saying to the magazine.

Trump and Biden are set to spar during their first debate at Case Western Reserve University in Cleveland on September 29.